Facebook workers have often omitted conservative political stories from the website’s “trending” list, the technology news site Gizmodo said in a report that sparked widespread comment on social media.

An unnamed former Facebook employee told Gizmodo that workers “routinely suppressed news stories of interest to conservative readers,” according to Gizmodo, while “artificially” adding other stories into the trending list.

Facebook told Reuters on Monday that there are “rigorous guidelines in place” to maintain neutrality and said that these guidelines do not prohibit any news outlet from appearing in trending topics.

Facebook did not respond directly though to questions about whether employees had suppressed conservative-leaning news.

“These guidelines do not permit the suppression of political perspectives. Nor do they permit the prioritisation of one viewpoint over another or one news outlet over another,” a spokesperson for Facebook said.

The report alarmed some social media users, with several journalists and commentators criticising Facebook for alleged bias.

“Well, you go to Hell, Facebook,” tweeted Kyle Feldscher (@Kyle_Feldscher), a reporter at the Washington Examiner, a conservative-leaning publication.

“For anyone who cares about press freedom, this is frightening stuff,” tweeted Bloomberg Editor Bill Grueskin (@BGrueskin), with a link to Gizmodo’s story.

“Former Facebook Workers” quickly became one of the top-ten trending topics on Twitter in the US after the Gizmodo story broke.

The Conservative Political Action Conference (CPAC), one of the groups reportedly blocked from Facebook’s trending list, said it would closely monitor how the claims against Facebook unfold.

“If we can confirm that the allegations are true and accurate, that would be disappointing,” said CPAC Communications Director Ian Walters, who added that he was sensitive to the fact that the claims are as of yet unconfirmed.

A post on Facebook’s help center said that the “trending” section of the site “shows you topics that have recently become popular on Facebook.” It lists “engagement, timeliness, Pages you’ve liked and your location” as some of the factors that determine what trends show up for each Facebook user.

Facebook users can also manually remove certain topics from their trending list.

On the right side of your Facebook window is the “trending” news module. Maybe you use it! It might not look like much, with its little blue arrows and oddly robotic cadence, but in practice it’s one of the most powerful little rectangles in all of online publishing. Not only does it provide a firehose of traffic to media companies lucky enough to land their stories there—say, GQ.com!—but it’s said to function as an impartial facilitator of news that tells Facebook users what they need to know.

Or at least that was the thought. Today, Gizmodo published a damning report in which former Facebook contractors in charge of regulating the trending section were told to suppress news from right-wing outlets. The future of media, folks:

Facebook workers routinely suppressed news stories of interest to conservative readers from the social network’s influential “trending” news section, according to a former journalist who worked on the project. This individual says that workers prevented stories about the right-wing CPAC gathering, Mitt Romney, Rand Paul, and other conservative topics from appearing in the highly-influential section, even though they were organically trending among the site’s users.

Furthermore, it appears that curators were instructed to not include stories that might be damaging to Facebook, the company:

“We were always cautious about covering Facebook,” said another former curator. “We would always wait to get second level approval before trending something to Facebook. Usually we had the authority to trend anything on our own [but] if it was something involving Facebook, the copy editor would call their manager, and that manager might even call their manager before approving a topic involving Facebook.”

To be clear, every news outlet has its inherent biases—GQ included! (Longtime readers know we haven’t been “sticking to fashion” since, oh, 1957?) The problem in this case is when a broadcast tool as powerful as Facebook bills itself as a neutral feed, when in actuality its liberal biases play an important role in shaping exactly what readers encounter. (And that’s to say nothing of the way its editorial tendencies pave the way for, say, the clickbait economy. But that’s a story for another time!)

Update 4:21 P.M.: A Facebook representative sent us the following statement:

“We take allegations of bias very seriously. Facebook is a platform for people and perspectives from across the political spectrum. Trending Topics shows you the popular topics and hashtags that are being talked about on Facebook. There are rigorous guidelines in place for the review team to ensure consistency and neutrality. These guidelines do not permit the suppression of political perspectives. Nor do they permit the prioritization of one viewpoint over another or one news outlet over another. These guidelines do not prohibit any news outlet from appearing in Trending Topics.”